3.5.3Organic Chemistry
Distillation of an Organic Product — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 22 of 55
Part 1 of 6Why Distil the Product?
Ethanal is formed in the same flask as ethanol and dichromate, so it must be removed before it is oxidised further.
Boiling points
- Ethanal boils at about 20 °C while ethanol boils at about 78 °C, so gentle heating drives off the ethanal first.
Preventing over-oxidation
- Removing ethanal as it forms stops it staying in contact with the oxidant and turning into ethanoic acid.
Hydrogen bonding
- Ethanol hydrogen bonds to itself but ethanal cannot, which is why the aldehyde is much more volatile.
Reaction
- CH₃CH₂OH + [O] → CH₃CHO + H₂O, with orange dichromate ions turning green as they are reduced.
Part 2 of 6Method
The apparatus is set up for distillation rather than reflux so the product leaves the flask.
1
Place acidified potassium dichromate(VI) in a pear-shaped flask and stand the flask in cold water because the mixing is exothermic.
2
Add the ethanol dropwise from a pipette, keeping the mixture cool so that the volatile ethanal is not lost.
3
Add anti-bumping granules and attach the flask to a still head, thermometer and condenser with water flowing upwards.
4
Heat gently with an electric heater and collect the distillate in a receiver kept cool, since ethanal is very volatile.
5
Stop collecting when a few cm³ of distillate have been obtained and keep the sample stoppered for testing.
Part 3 of 6Testing the Distillate
Testing the Distillate
Making Tollens’ reagent
- Add a few drops of sodium hydroxide to silver nitrate solution to form a brown precipitate,
- then add dilute ammonia until it just redissolves.
The reagent
- This gives colourless [Ag(NH₃)₂]⁺, which acts as a mild oxidising agent and must be freshly made.
Positive result
- On warming with the distillate, Ag⁺ is reduced to Ag, which coats the inside of the tube as a silver mirror.
Reduction half-equation
- [Ag(NH₃)₂]⁺ + e⁻ → Ag + 2NH₃ shows silver being reduced while ethanal is oxidised to ethanoic acid.
Part 4 of 6Safety and Evaluation
Safety and Evaluation
Heating
- Ethanol and ethanal are flammable, so an electric heater or hot water bath is used and not a naked flame.
Hazards
- Acidified dichromate is corrosive and toxic, so wear gloves and eye protection and work in a well-ventilated area.
Sources of loss
- Ethanal can escape as vapour, some may be oxidised to the acid, and some stays in the apparatus, all lowering yield.
Waste
- Silver-containing and chromium-containing residues are collected for disposal and Tollens’ reagent is not stored.
Part 5 of 6Worked Example
Question
- 4.60 g of ethanol (Mr = 46.0) is oxidised to ethanal (Mr = 44.0) and 2.64 g of ethanal is collected.
- Calculate the percentage yield.
- Moles of ethanol = 4.60 ÷ 46.0 = 0.100 mol.
- The equation shows 1 mol ethanol gives 1 mol ethanal, so the maximum is 0.100 mol.
- Theoretical mass of ethanal = 0.100 × 44.0 = 4.40 g.
- Percentage yield = (2.64 ÷ 4.40) × 100 = 60.0%.
Answer The percentage yield of ethanal is 60.0%.
Part 6 of 6Exam Checklist & Tips
3.5.3 – Key Points for the Exam
- Distillation Separation of liquids by boiling and condensing the vapour of the lower boiling one.
- Anti-bumping granules Small pieces added to a liquid to give smooth boiling.
- Tollens’ reagent An alkaline solution of diamminesilver(I) ions used to detect aldehydes.
- Percentage yield Actual mass obtained divided by the theoretical mass, multiplied by 100.
Common Mistakes
- Adding Tollens’ reagent to the dichromate flask instead of testing the collected distillate.
- Heating with a Bunsen flame, which is unsafe because the reagents and product are flammable.
- Adding all the ethanol at once; the reaction is exothermic and ethanal vapour is lost.
- Using reflux when the aim is to collect ethanal, which would return it to the flask and form ethanoic acid.
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